Surface Proton Conduction over Catalyst Support via Chemically Grafted Groups

Abstract

The proton conduction within a catalyst layer is one of the critical factors affecting the performance of membrane electrode assembly in polymer electrolyte fuel cells. In this work, a simple and effective approach for providing surface proton conduction over carbon supports was developed utilizing chemically grafting SO3H and COOH groups covalently bonded onto carbon surface. A method for accurately measuring the proton conductivity within a catalyst layer was also developed which physically excluded the conductivity contribution from the membrane. This method has several advantages: 1) providing easy sample preparation with high reproducibility, 2) allowing the control of measurement conditions such as gas flow rate, relative humidity and temperature to mimic the exact fuel cell operating conditions. The proton and electron conductivities of the catalyst layers made with and without functional groups at different relative humidifies were characterized with a specially designed 4-probe cell using the AC impedance spectroscopy technique. The results clearly demonstrated that the introduction of functional groups (i.e., SO3H and COOH) did result in a significant improved surface proton conduction over carbon surface and the resulted conductivity depends on the relative humidity, temperature and porosimetry of the carbon blacks.